At least three prior art references describe improvements made to linear voice coil actuators by removing some material from one or more parts of the magnetic circuit. U.S. Pat. No. 5,808,379, issued to Zhao and assigned to Kulicke and Soffa Industries, Inc. shows such an improvement: at least one air gap 44 is provided in at least one of the pole pieces. See FIG. 1, herein. (FIG. 1 is a reproduction of FIG. 5 from U.S. Pat. No. 5,808,379, but in which reference numbers have been removed to avoid confusion with reference numbers appearing in FIG. 2 of the subject application.) The stated improvement is an increase in force for a fixed current level due to reduced armature reaction.
Another prior art actuator is depicted in FIG. 2 and described in U.S. Pat. No. 5,113,099, issued to Wong, et al., and assigned to Maxtor Corporation. (FIG. 2 is a reproduction of FIG. 3 from U.S. Pat. No. 5,113,099.) According to this patent, the core 35 has an opening 37 that improves the rise time of the actuator, while additional openings 36 are made to provide heavy saturation of the core to make its magnetic characteristic approach that of air.
Yet another prior art actuator, depicted in FIG. 3, herein, is described in U.S. Pat. No. 5,677,963. issued to Morcos, et al., and assigned to the assignee of the subject application. According to this patent, a hole 109 is drilled through the core 108 in its center. Because of the hole, the flux created by the permanent magnets is not affected, but the flux created by the current in the coil (armature reaction) is substantially reduced. One of the positive effects of this measure is reduction of the force imbalance for any given position of the coil and two different directions of current in the coil.
All of the above three patents substantially describe rectangular voice coil actuators, although U.S. Pat. No. 5,677,963. mentions that the core could be cylindrical. However, the above mentioned patent describes field plates connected to end plates in a fashion which can be interpreted to mean that only two magnets or two sets of magnets are attached to the parallel plates and only two sides of the coil are involved in creating the force.
Despite improvements in actuator performance achieved by these earlier efforts, there continues to be a need for improvement in force imbalance as a function of the position of the coil and current direction in the coil.